In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
High-Speed Uplink Packet Access (HSUPA) is a 3G mobile telephony protocol, also known as Enhanced Uplink (EUL). The specifications for HSUPA are included in Universal Mobile Telecommunications System Release 6 standard published by 3GPP. The technical purpose of the Enhanced Uplink is to improve the performance of uplink dedicated transport channels, e.g., to increase capacity and throughput and reduce delay. HSUPA uses an uplink enhanced dedicated channel (E-DCH) on which it employs link adaptation methods similar to those employed by High-Speed Downlink Packet Access (HSDPA). HSUPA, among other technologies, is discussed in Dahlman, Erik, et al, 3G Evolution: HSPA and LTE for Mobile Broadband, Academic Press, 2008, ISBN: 978-0-12-374538-5, which is incorporated herein by reference in its entirety.
HSUPA uses a packet scheduler which is located in the NodeB. The scheduler forms part of a unit known as a NodeB MAC-e, which unit is responsible for support of fast hybrid ARQ transmissions and scheduling. The UE also has a MAC-e unit; the UE MAC-e unit is responsible for selecting the data rate within limits set by the scheduler in the NodeB MAC-e.
HSUPA operates on a request-grant principle according to which the UEs request permission to send data and the scheduler in the NodeB decides when and how many UEs will be allowed to do so. That is, the scheduler in the NodeB receives scheduling requests from the UE when the UE desires to transmit at a higher data rate than currently allowed. A request for transmission from the UE contains data about the state of the transmission buffer in the UE and the queue at the UE, and the UE's available power margin. The NodeB for the serving cell then makes a scheduling decision and, if the decision is favorable, responds with a scheduling grant.
At the Physical Layer, HSUPA introduces new channels E-AGCH (Absolute Grant Channel), E-RGCH (Relative Grant Channel), F-DPCH (Fractional-DPCH), E-HICH (E-DCH Hybrid ARQ Indicator Channel), E-DPCCH (E-DCH Dedicated Physical Control Channel) and E-DPDCH (E-DCH Dedicated Physical Data Channel). E-DPDCH is used to carry the E-DCH Transport Channel. That is, the E-DCH is mapped to a set of channelization codes known as E-DCH Dedicated Physical Data Channels (E-DPDCHs). Depending on the instantaneous data rate, the number of E-DPDCHs and their spreading factors are both varied. The E-DPCCH is used to carry the control information associated with the E-DCH.
As mentioned above, for Enhanced Uplink the scheduler is located in the NodeB, and the NodeB scheduler controls when and at what rate a UE is allowed to transmit, thereby controlling the amount of interference affecting other users at the NodeB. This can be seen as controlling each UE's consumption of common radio resources, which in the case of Enhanced Uplink is the amount of tolerable interference. In the uplink, the total amount of tolerable interference is defined as the average interference over all the RX antennas. The amount of common uplink resources a terminal is using depends on the data rate used. Generally, the higher the data rate, the larger the required transmission power and thus the higher resource consumption.
A relative measure of total interference is Rise over Thermal (RoT), i.e. total interference relative to thermal noise. The term “noise rise” or “rise-over-thermal” is often used when discussing uplink operation. Noise rise, defined as (I+N0)/N0 where N0 and I are the noise and wideband interference, respectively, is a measure of the increase in interference in the cell due to transmission activity. The uplink scheduler in the NodeB needs to keep the noise rise within acceptable limits.
Uplink load control adjusts the load headroom for a cell so that the measured RoT is controlled towards a target RoT. In uplink transmissions an inner loop power control (ILPC) (e.g., “inner control loop”) enables the UE to adjust its output power in accordance with one or more TPC (transmit power control) commands received in the downlink. The target RoT is determined primarily by factors such as network dimensioning for coverage considerations. The TPC (transmit power control) commands are issued by the aforementioned NodeB for maintaining quality of control channel DPCCH. While the power control and scheduler are different functions of the eNodeB, there is some interaction between power control and load control/scheduling.
The uplink scheduler in the NodeB allocates the available UL load to the scheduled users who require higher uplink bit-rate and reduces the granted uplink bit-rate of some scheduled users when the system is overloaded, e.g., when there is a RoT peak. However, due to the large time delay in uplink load control and scheduler, large RoT oscillation can occur, either above or below than the RoT target, and the RoT peak can last a long time before the RoT is reduced to an acceptable level. Such time delay may be occasioned by numerous factors, including RoT measurement delay; Node B processing delay; and grant transmission and processing delay, etc.
A “load factor” represents the portion of uplink interference that a certain channel of a certain user terminal generates, which is defined as the interference due to the channel of that user terminal divided by the total interference. The total load factor of different channels is equal to the sum of load factors of the different channels. Uplink load estimation estimates the load that has been or will be generated in each cell from different channels. Power based load estimation means load estimation according to the load factor definition as described above. A benefit of power based load estimation is that it is receiver independent and can naturally capture the receiver gain of various types of receivers.
In order to reduce the RoT peak levels and suppress the RoT peaks quickly, Fast Congestion Control (FCC) has been proposed. For example, in WO/2001/080575, entitled “CELLULAR COMMUNICATIONS SYSTEM/METHOD WITH UPLINK INTERFERENCE CEILING; and US Patent Publication 2003/0003921 A1, entitled “Method for Traffic Load Control in a Telecommunication Network”, both incorporated herein by reference in their entireties, it is proposed that TPC down commands are sent to selected users when the measured RoT exceeds the target level. For a selected UE, the Fast Congestion Control (FCC) procedure is executed as follows: (1) if the TPC generated by the inner loop power control (ILPC) is TPC DOWN command, the same TPC DOWN command is sent to the UE without changes; (2) if the TPC generated by the inner loop power control is TPC UP command, FCC changes the TPC command from UP to DOWN and this TPC DOWN command is sent to the UE. This is referred to hereinafter as the “forced TPC down command”.
As Fast Congestion Control (FCC) reacts much faster than the uplink load control and the scheduler, RoT can be better controlled and uplink load can be more efficiently utilized. Therefore, Fast Congestion Control (FCC) can be a low cost implementation for considerable gain in the future.
A drawback of Fast Congestion Control (FCC) is that the block error rate (BLER) of the users targeted by FCC will increase substantially. This may cause problem especially for users with relatively high QoS requirement.
With interference cancellation (IC) the signal to noise interference ratio (SINR) can be significantly improved, and thus also an evident improvement in data rate. Both E-DPDCH and DPCCH can benefit from interference cancellation (IC), but it is important to realize the first initial channel estimate based on only DPCCH must be good enough to start the whole interference cancellation (IC) process, and that channel estimate is before any IC. Thus, the DPCCH quality must be good enough and a bad DPCCH quality can negatively impact the benefit we can get from interference cancellation (IC).
During data transmission in general, a receiver needs to know what type of Transport Format (TF) is valid for each transport channel, e.g., the number of bits that will be transmitted on the transport channel during a transmission time interval (TTI). A transport channel may have several different possible transport formats, which each transport format having a different Transport Format Identifier (TFI). A dynamic part of the Transport Format defines the Transport Block Size and a Transport Block Set Size (how many Transport Blocks can be delivered in on TTI); a static part of the Transport Format defines such parameters as the TTI, coding type and size; size of CRC, etc. Given the fact that plural transport channels may be used at one time, a parameter known as the Transport Format Combination (TFC) is used to express the TFIs for the plural channels. For example, the Transport Format Combination (TFC) provides information on how many bits (Transport Blocks) of each transport channel are transmitted in the next TTI.
For the Enhanced Uplink and its E-DCH, the possible E-TFCs, i.e., the possible transport block sizes, are predefined in the specifications similar to HS-DSCH. At connection setup, a set of up to eight reference E-TFCs (Enhanced Transport Format Combinations), their transport block size and quantization amplitude ratios, are signaled to the UE. Then, during an HSUPA connection, the UE can calculate the needed transmission power for each E-TFC based on the referenced E-TFCs and its quantized power ratio. At each TTI boundary, the UE determines the state of each E-TFC based on the E-TFC's required transmit power versus the maximum allowed UE transmit power. Once the E-TFC is selected along with all of the signaled parameters, the uplink is completely configured and the data rate for the next transmission is known.
As mentioned above, the NodeB scheduler sends scheduling grants to the UE. A scheduling grant includes an index value, with the index value sent in the scheduling grant being one of several possible index values stored in a table, each stored index value in turn being matched or paired with an associated power offset. Each power offset stored in the table is an indication of how much power relative to power of the DPCCH the wireless terminal is allowed to use for transmission of the E-DCH when the associated index is received in the scheduling grant. The grants may be either absolute grants or relative grants. Absolute grants, sent on the AGCH, provide an absolute limitation of the maximum amount of uplink power resources that the UE may use. The grants also inherently limit the uplink resources since there is a relation between the E-TFC, the number of codes, spreading factor, and modulation that the wireless terminal may use. Absolute grants are sent to the UE usually at the start of a HSUPA connection. The absolute grant value indicates the maximum E-DCH traffic to pilot power ratio (E-DPDCH/DPCCH) that the UE is allowed to use in the next transmission. Relative grants, sent on the E-RGCH, increase or decrease the power compared to the previously used to value. The relative grants can be sent every scheduling period, on a dedicated channel. Relative grants typically change the E-DPDCH power in small amounts relative to the previous value. The UE uses the information in the absolute and relative grants to calculate its “serving grant”. This is serving grant is updated in accordance with the scheduling period. The serving grant permits the UE to calculate its maximum power to use to transmit data on the E-DPDCH(s). The UE has been informed at connection set up how much power is needed to use each physical channel combination, so the UE knows what is the maximum block size it can transmit for each TTI.
The power needed for an E-DCH transmission is calculated from two power offsets relative to the power for the DPCCH. One power offset is associated with each E-TFC and one power offset is associated with the hybrid ARQ profile. The resulting transmitter power is then calculated by adding these two power offsets to the DPCCH power. When the required transmitter power for different E-TFCs has been calculated, the UE can calculate which E-TFCs are possible to use from a power perspective. The UE then selects the E-TFC by maximizing the amount of data that can be transmitted given the power constraint and the scheduling grant.
Usually the maximum allowed power of a UE is determined by the maximum transmission power of a UE, for example 20 dBm, and power back off requirements, for example power back off for higher order modulation to reduce the PAPR (Peak to Average Power Ratio).
As stated above, the maximum allowed power is one of the limiting factors when the UE decides its transport format. The other two limiting factors are available data in the UE buffer and the scheduling grant set by the Node B. If UE is power limited, which means the power limit is the tightest limitation among all the limiting factors, the E-TFC selection in power limited scenario is illustrated in FIG. 1A. FIG. 1B, on the other hand, illustrates an example E-TFC selection in a non-power-limited scenario. The UE can check if it is in a power limited scenario by comparing selected E-TFC using power limit and E-TFCs selected according to the grant limit and data buffer limit.
PCT/CN2011/001888, which concerns BLER Based Load Control Improvement and which is incorporated herein by reference, proposes a method wherein Block Error Rate (BLER) statistics are fed into the load control/scheduler so that the uplink radio resource utilization is enhanced by more aggressive utilization of Fast Congestion Control (FCC) while the BLER of the served EUL users is controlled within a predefined acceptable range by adjusting the load headroom/granted power offset accordingly.
With Fast Congestion Control (FCC), the DPCCH power is reduced by the inner loop power control (ILPC) in order to mitigate the RoT rush. However, there is a tradeoff between the better controlled RoT and the increased retransmission rate. With an aggressive Fast Congestion Control (FCC) algorithm, the uplink load can be well controlled, but there can be a very high retransmission rate due to the low DPCCH power which is not desirable especially for users with delay sensitive traffic.
This problem is more critical with power-based load estimation, since a decreased DPCCH power by Fast Congestion Control (FCC) will lead to a lower estimated DPCCH load, and consequently a higher granted power offset, which in turn leads to even higher BLER and consequently even higher retransmission rate. In other words, Fast Congestion Control (FCC) cannot be set too aggressively and some load margin still needs to be reserved especially with power based load estimation. Besides, as described above, a bad DPCCH quality can negatively impact the benefits obtained from interference cancellation (IC).
A proposed solution tends to retrieve the E-TFC grant when high block error rate is observed. However, there is considerable delay from when a new grant is scheduled to the new grant takes effect. Because of the long delay the negative effect due to interaction between FCC and power based load estimation cannot be effectively mitigated, and the negative impact on interference cancellation (IC) may still exist. For this and other reasons, Fast Congestion Control (FCC) may not be utilized too aggressively and some load margin still needs to be reserved. Moreover, the scarce downlink code resource also makes it is difficult to retrieve the grants of many users.